System for measuring stress in downhole tubulars

ABSTRACT

An apparatus for evaluating the possibility of casing failure in a borehole measures azimuthal changes in a property responsive to azimuthal changes in axial stress. The measurements may be of acoustic velocity, magnetic permeability, and Barkhausen noise.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 11/622,101 filed on Jan. 11, 2007.

BACKGROUND OF THE DISCLOSURE

1. Field of the Disclosure

The present disclosure provides an apparatus and method for testing thestructural integrity of tubing and casings used in a borehole. Inparticular, the present disclosure discusses an apparatus and methodusing ultrasonic waves to estimate the stress on tubulars in a boreholeenvironment.

2. Description of the Related Art

The environmental conditions encountered by production casing and tubingused in hydrocarbon recovery can result in stress buildup in the tubing.This stress in the tubing may come from pressure and temperaturevariations during production, movement of the formation due to pressuredepletion, “flow” of salt formations, etc. This stress may eventuallylead to casing or tubing collapse or shear, rendering the wellinoperable. Prior art methods have generally involved waiting for thebuildup of this stress to a point where mechanical deformation occursbefore the stress can be detected.

Stress buildup may also occur in a drillstring during the drilling of aborehole. During drilling operations, it is not uncommon for thedrillstring to get stuck. To recover the stuck pipe, it is firstrequired to determine the upper most ‘free’ point of the drillpipe. Thisis done by measuring the torque and/or pull induced from the surface orthe physical stretching of the drillpipe due to this torque or pull.

Stress in a casing or tubing may be in the form of an axial load,circumferential torque, or a bending moment. Although stresses areapplied on the drilling equipment while in use in the boreholeenvironment, testing for wear typically occurs uphole or in alaboratory, often by observing the residual stress on the mandrel fromits use. In general, when a stress is applied to a material and thenremoved, a residual stress remains on the material. This residual stressis often observed by checking for atomic dislocations at the crystallinelevel of the material and can be used to determine properties related tothe structural integrity of the material. Various methods have beendesigned to observe residual stress on materials, including X-raydiffraction techniques, determining magnetic permeability, andultrasonic testing.

Changes in ultrasonic wave propagation speed, along with energy lossesfrom interactions with materials microstructures are often used tonondestructively gain information about properties of the material. Anultrasonic wave may be created in a material sample, such as a solidbeam, by creating an impulse at one region of the sample. As the wavepropagates through the sample, stresses and other material changes ordefects affect the wave. Once the affected wave is recorded, the natureof the stresses of the material can be determined. Measurements of soundvelocity and ultrasonic wave attenuation can be related to the elasticproperties that can be used to characterize the texture ofpolycrystalline metals.

Velocity measurements are of interest in longitudinal waves propagatingin gases, liquids, and solids. In solids, transverse (shear) waves arealso of interest. The velocity of a longitudinal wave is independent ofa sample's geometry when the dimensions at right angles to the sampleare large compared to the sample area and to the wavelength. Thevelocity of a transverse wave is affected little by the physicaldimensions of the sample. The relationship between stress and velocityhas been discussed for example by Cantrell and Chern, “Relative SlopeInvariance of Velocity-Stress and Strain-Stress Curves,” UltrasonicsSymposium, 1981.

Measurement of ultrasonic velocity is performed by measuring the time ittakes for a pulse of ultrasound to travel from one transducer to another(pitch-catch scenario) or return to the same transducer (pulse-echoscenario). Another measurement method compares the phase of the detectedsound wave with that of a reference signal, wherein slight changes inthe transducer separation are seen as slight phase changes, from whichthe sound velocity can be calculated. These methods are suitable forestimating acoustic velocity to about 1 part in 100. Standard practicefor measuring velocity in materials is detailed in American Society forTesting and Materials (ASTM) Publication E494. Residual stressmeasurements in cylinders have been discussed for example by Frankel etal., “Residual Stress Measurement in Circular Steel Cylinders,”Ultrasonics Symposium, 1983.

In petroleum exploration, time spent raising and lowering a drillingapparatus from and into a borehole is time that could otherwise be usedin exploration and is thus costly. Historically, stress on a tubularcontaining drilling equipment used in a borehole has only beendetermined by looking for actual physical movement of the tubular (i.e.,freepoint indicators) or by physical distortion of the tubular (i.e.,casing inspection). Thus, it is desirable to perform stress testing of adrilling apparatus obtaining measurements downhole.

SUMMARY OF THE DISCLOSURE

One embodiment of the disclosure is an apparatus for identifying a shearstress in a tubular within a borehole. The apparatus includes a loggingtool including at least one transducer configured to generate a wave inthe tubular. The apparatus also includes a plurality of receiversazimuthally distributed in the logging tool. Each of the receivers isconfigured to produce a signal responsive to the generated wave and anazimuthal stress distribution in the tubular. The apparatus alsoincludes at least one processor configured to determine from the signalfrom each of the plurality of receivers an indication of the azimuthalstress distribution in the tubular, and estimate from the indication ofthe azimuthal stress distribution a value of the bending stress in thetubular.

Another embodiment of the disclosure is a method of identifying a shearstress in a tubular within a borehole. The method includes using atleast one transducer on a logging tool for generating a wave in thetubular, and using each of a plurality of receivers azimuthallydistributed on the logging tool for producing a signal responsive to thegenerated wave and an azimuthal stress distribution in the tubular. Themethod further includes determining from the signal from each of theplurality of receivers an indication of the azimuthal stressdistribution in the tubular, and estimating from the indication of theazimuthal stress distribution a value of the bending stress in thetubular.

Another embodiment of the disclosure is a computer-readable medium foruse with an apparatus for identifying a shear stress a tubular within aborehole. The apparatus includes at least one transducer on a loggingtool configured to generate a wave in the tubular, and a plurality ofreceivers azimuthally distributed on the logging tool, each of theplurality of receivers configured to produce a signal responsive to thegenerated wave and an azimuthal stress distribution in the tubular. Themedium includes instructions that enable at least one processor todetermine from the signal from each of the plurality of receivers anindication of the azimuthal stress distribution in the tubular, andestimate from the indication of the azimuthal stress distribution avalue of the bending stress in the tubular. The medium may include aROM, an EPROM, and EEPROM, a flash memory and/or an optical disk.

BRIEF DESCRIPTION OF THE DRAWINGS

For a detailed understanding of the present disclosure, reference shouldbe made to the following detailed description of the disclosure, takenin conjunction with the accompanying drawing and in which:

FIG. 1 is a schematic illustration of a wireline logging system;

FIG. 2A is an illustration of a logging tool according to the presentdisclosure within a cased borehole with poor cementing;

FIG. 2B shows an exemplary pad containing an array of transducerscapable of performing the method of the present disclosure;

FIGS. 3A-E shows various practical transducer configurations that may beused on a material and resultant forces on the surface of the materialfor producing acoustic pulses;

FIG. 4 is a schematic illustrations of two EMATs configured to generateshear-waves in two different directions;

FIG. 5 illustrates bending of casing resulting from subsidence of theocean bottom upon which a platform is located;

FIGS. 6A, 6B shows the differential axial stresses resulting fromtorsion on a casing;

FIGS. 7A, 7B and 7C show an embodiment of the disclosure in whichmagnetic permeability changes may measured; and

FIG. 8 shows an embodiment of the disclosure in which noise resultingfrom the Barkhausen effect is used.

DETAILED DESCRIPTION OF THE DISCLOSURE

The present disclosure is discussed with reference to specific logginginstruments that may form part of a string of several logginginstruments for conducting wireline logging operations. It is to beunderstood that the choice of the specific instruments discussed hereinis not to be construed as a limitation and that the method of thepresent disclosure may also be used with other logging instruments aswell.

FIG. 1 shows a logging tool 10 suspended in a borehole 12 thatpenetrates earth formations such as 13, from a suitable cable 14 thatpasses over a sheave 16 mounted on drilling rig 18. By industrystandard, the cable 14 includes a stress member and seven conductors fortransmitting commands to the tool and for receiving data back from thetool as well as power for the tool. The tool 10 is raised and lowered bydraw works 20. Electronic module 22, on the surface 23, transmits therequired operating commands downhole and in return, receives data backwhich may be recorded on an archival storage medium of any desired typefor concurrent or later processing. The data may be transmitted inanalog or digital form. Data processors such as a suitable computer 24,may be provided for performing data analysis in the field in real timeor the recorded data may be sent to a processing center or both for postprocessing of the data. The system includes novel features that arediscussed below.

FIG. 2A is a schematic external view of a borehole system according tothe present disclosure. The tool 10 comprises the arrays 26 and issuspended from cable 14. Electronics modules 28 and 38 may be located atsuitable locations in the system and not necessarily in the locationsindicated. The components may be mounted on a mandrel 34 in aconventional well-known manner. In an exemplary assembly, the outerdiameter of the assembly is about 5 inches and about fifteen feet long.An orientation module 36 including a magnetometer and an accelerometeror inertial guidance system may be mounted above the imaging assemblies26 and 32. The upper portion 38 of the tool 10 contains a telemetrymodule for sampling, digitizing and transmission of the data samplesfrom the various components uphole to surface electronics 22 (FIG. 1) ina conventional manner. If acoustic data are acquired, they arepreferably digitized, although in an alternate arrangement, the data maybe retained in analog form for transmission to the surface where it islater digitized by surface electronics 22.

FIG. 2B shows an exemplary pad containing transducers capable ofperforming the method of the present disclosure. Pad 40 includes one ormore acoustic sensors 45. In one embodiment of the disclosure, theacoustic sensors comprise electromagnetic acoustic transducers (EMATS)assembled in a pattern to obtain measurements of ultrasonic velocitiesfor the purpose of determining a stress on a material. The pad 40 isattached to the mandrel 34 of FIG. 2A by way of supports 42. The patternof EMATS shown in FIG. 2B is only an example of many possibleconfigurations that may be used.

In another embodiment of the disclosure, the sensors may be disposed ontwo or more vertically spaced apart pads. Such an arrangement makes iteasier to make axial measurements as a described below.

The present disclosure generally uses orthogonal acoustic velocitymeasurements in the steel tubulars to determine in-situ stress. In onepossible embodiment, the velocity of a vibrational (acoustic) wavetraveling axially in a casing is compared to the velocity of a similarwave traveling circumferentially at substantially the same point in thecasing. Differences in the resulting measured velocities indicate eithertorque or axial stress in the casing. With a more complex arrangementusing segmented circumferential or axial measurements, differences inaxial stress around the circumference of the casing may indicate bendingor crushing loads being applied to the casing by the formation. Also,localized stress measurements made in the area of casing corrosion ormechanical defects can be used to predict potential points of casingrupture. Since the properties of casing steel may vary, the use oforthogonal measurements is critical to identifying changes caused bystress from background changes in materials.

Measurement of acoustic travel time may be substituted with alternativemeasurements that are affected by casing stress. One alternativemeasurement might be magnetic permeability. The angle between the twomeasurements may be something other than orthogonal. A 90° angle,however, maximizes sensitivity of the measurement.

Measurements of stress in casing or tubing downhole have multiplepotential uses. These uses potentially include casing deformation,freepoint indicators, and formation stresses (as transferred to thecasing). The disclosed method offers a potential method of making anabsolute stress measurement in a casing or tubing.

The present disclosure discusses an apparatus and method for performingacoustic testing on a casing or tubular. An ultrasonic wave can beproduced at one location on the tubular and the wave can later bedetected at the same or another location on the tubular. One way tocreate ultrasound within a material is via an EMAT. An EMAT comprises amagnetic element, such as a permanent magnet, and a set of wires. Ingeneral, the EMAT is placed against the material to be tested such thatthe set of wires are located between the magnetic element and thematerial to be tested. When a wire or coil is placed near to the surfaceof an electrically conducting object and is driven by a current at adesired ultrasonic frequency, eddy currents are induced in a nearsurface region. If a static magnetic field is also present, thesecurrents experience a Lorentz force of the form{right arrow over (F)}={right arrow over (J)}×{right arrow over(B)}  (1)where {right arrow over (F)} is a body force per unit volume, {rightarrow over (J)} is the induced dynamic current density, and {right arrowover (B)} is the static magnetic induction. Thus the Lorentz forceconverts the electrical energy into a mechanical vibration, which can beused to test the material. Alternatively, EMATs may also be based on theuse of magnetostrictive properties of the casing/tubing.

Since no coupling device is used between the EMAT and the testedmaterial, the EMAT can operate without contact at elevated temperaturesand in remote locations. Thus EMATs can eliminate errors associated withcoupling variation in contact measurements and thereby provide precisevelocity or attenuation measurements.

The coil and magnet structure used in an EMAT can be designed to excitecomplex wave patterns and polarizations. FIGS. 3A-3F shows a number ofpractical EMAT configurations including a biasing magnet structure, acoil configuration, and resultant forces on the surface of the solid forproducing acoustic pulses using EMATS. The configurations of FIGS. 3A,3B, and 3C excite beams propagating normal to the surface of ahalf-space and produce, respectively, beams with radial, longitudinal,and transverse polarizations. The configurations of FIGS. 3D and 3E usespatially varying stresses to excite beams propagating at oblique anglesor along the surface of a component. These configurations are consideredfor illustrative purposes although any number of variations on theseconfigurations can be used.

FIG. 3A shows a cross-sectional view of a spiral coil EMAT configurationfor exciting radially polarized shear waves propagating normal to thesurface. Permanent magnet 301 and tubular 307 are separated by a spacecontaining a wire represented by one or more wires as shown as wiresegments 303 and 305. The wire segments 303 and 305 represent separategroups of wire segments carrying current in anti-parallel directions inthe manner illustrated in FIG. 3A, thereby exciting the radiallypolarized shear waves propagating normal to the surface.

FIG. 3B shows a cross-sectional view of a tangential field EMATconfiguration for exciting longitudinally polarized compressional wavespropagating normal to the surface. Permanent magnet 311 is placedagainst tubular to produce a magnetic field parallel to the surface. Amagnet such as the magnet 311 of FIG. 3B having a horseshoeconfiguration may be used. Wires segments 313 provide a current flowingbetween the magnetic poles perpendicular to the direction of the localmagnetic field of magnet 311. Wire segments 315 provide a currentflowing anti-parallel to the current in wire segments 313 in a regionexterior to the magnetic poles.

FIG. 3C shows a cross-sectional view of a normal field EMATconfiguration for exciting plane polarized shear waves propagatingnormal to the surface. The configuration comprises a pair of magnets 321and 323 assembled so as to provide two anti-parallel magnetic fields atthe surface of the tubular. The permanent magnets 321 and 323 areseparated from tubular 329 by a space containing one or more wires 325and 327 providing anti-parallel current.

FIG. 3D shows a cross-sectional view of a meander coil EMATconfiguration for exciting obliquely propagating L (long) or SV waves,Rayleigh waves, or guided modes (such as Lamb waves) of plates. Theconfiguration includes a permanent magnet and tubular separated by aspace containing wire segments such as one or more wires 333 and 335which provides current flowing in sequentially alternating directions.

FIG. 3E shows a cross-sectional view of a periodic permanent magnet EMATfor exciting grazing or obliquely propagating horizontally polarized(SH) waves or guided SH modes of plates. Multiple permanent magnets suchas magnets 341 and 343 are assembled so as to provide alternatingmagnetic polarities at the surface of the tubular. The magnetic assemblyand tubular are separated by a space containing a wire 345 that providesa current in a single direction.

For sheet and plate specimens experiencing applied or residual stress,the principal stresses σ_(a) and σ_(b) may be inferred from orthogonalvelocity measurements. Eq. (2) relates ultrasonic velocities to theprinciple stresses experienced in a sheet or plate:2ρV _(avg) [V(θ)−V(θ+π/2)]=σ_(a)−σ_(b)  (2).In Eq. (2), V_(avg) is the average shear velocity and p is a density ofa material. V(θ) and V(θ+π/2) are mutually perpendicular wave velocitiesas can be detected at a transducer. It is understood that velocitydifference V(θ)−V(θ+π/2) is maximized when the ultrasonic propagationdirections are aligned with the principal stress axes. The magnitude ofthis difference, along with the density and mean velocity can be used toestimate the principal stress difference.

FIG. 4 shows an arrangement of two EMATS 145A and 145B. The pad 40illustrated and FIG. 2B is not shown. When EMATS 145A and 145B are ofthe type shown in FIG. 3E, they will produce horizontally polarizedshear-wave propagating along the tool axis and circumferential to thetool axis, thus providing the necessary measurements for solving eqn.(2). Those versed in the art would appreciate that using an array oftransducers as shown in FIG. 2B, it would be possible to generatehorizontally polarized shear waves propagating in different directions.The EMATs, in addition to acting as transmitters, can also act asreceivers, so that by having two EMATs with the same polarization atdifferent spatial positions, it is possible to determine the velocity ofpropagation of the wave. In addition, by having such transducers mountedon different pads on the downhole logging to it is possible to makemeasurements of the stress differences circumferentially around theborehole.

By using transducers of the type shown in FIG. 3B it would be possibleto make measurements of compression velocity at different azimuthalpositions along the borehole. Variations in this velocity are indicativeof circumferential variations of the stress. The same is true usingtransducers of the type shown in FIG. 3C. But using transducers of thetype shown in FIG. 3D it would be possible to generate Rayleigh waves onland waves along the surface of the tubular.

In addition, those versed in the art would recognize that the velocityof propagation of a vertically polarized shear-wave may differ from thevelocity of propagation of the horizontally polarized shear-wave in thesame direction. This difference may also be indicative of the stress inthe tubular. Such measurements may be obtained by using transducers ofthe type shown in FIGS. 3D and 3E.

In one embodiment a velocity of an acoustic wave traveling axially inthe casing is compared to the velocity of a similar wave travelingcircumferentially at substantially the same point in the casing.Differences in the measured velocities are indicative of torque or axialstress in the casing. With a more complex arrangement using segmentedcircumferential or axial measurements made with pad-mounted EMATs,differences in axial stress around the circumference of the casing areindicative of bending a crushing load being applied to the casing by theformation. Localized test measurements made in the area of casingcorrosion or mechanical defects are used to predict potential points ofcasing failure. As would be known to those versed in the art, suchcasing corrosion or mechanical defects would produce changes in thestress field. All of these use measurements having orthogonal directionof propagation or orthogonal polarization or both. Properties of casingssteel may vary, so that the use of such measurements is important inidentifying changes caused by stress from changes caused by differencesin the steel.

Turning now to FIG. 5, an important application of the methods of stressdetermination discussed above is illustrated. Shown is a productionplatform 501 resting on an initial position of the ocean bottom 505. Thesurface of the ocean is depicted by 503. Over time, production ofhydrocarbons from the subsurface commonly leads to subsidence of theocean bottom. The new positions of the ocean bottom and the productionplatform are denoted by 505′ and 501′ respectively. As an example,subsidence of the ocean bottom at the Ekofisk field in the Norwegianportion of the North Sea was 5.5 m after 25 years of production andcontinuing at the rate of 0.5 m/year. As a result of this subsidence, anextensive portion of the production casing 511 may be deformed, thedeformed position being denoted by 511′. The portion of the casing thatis relatively undisturbed is denoted by 513.

Those versed in the art would recognize that such large displacements ofthe casing introduce large stresses and can lead to significantweakening and potential catastrophic failure of the casing. For thisreason, it is important to be able to identify the stresses before theyget to the point where the casing is near its yield point.

One embodiment of the disclosure envisages using acoustic velocitymeasurements of the type discussed above to identify the stresses in thecasing. FIG. 6A shows an exemplary casing 511. The effect of thesubsidence is to subject the casing to a bending or shear stress such as603. Such a bending stress is easily seen to occur if the casing isinitially not vertical, but may also occur when the casing is initiallyvertical. In the latter case, buckling of the casing is the dominantmode of deformation, but the local bending or shear stress is still thedetermining factor for casing failure.

Shown schematically in FIG. 6B are the axial stresses resulting from thebending stress 603. In the right half of the casing, the stress may beextensional while the left half of the casing, the stress may becompressional. The stresses increase with radial distance from thecenter of the casing, depicted by the size of the x's and o's. In anazimuth that is 90° to that shown, the axial stresses will be zero. Itshould be noted that the terms “compressional” and “extensional” are tobe construed in a relative sense: what is important is the difference inaxial stress between the sides of the casing that are azimuthally 180°apart. Thus, using the transducer arrangements described above, it ispossible to estimate changes in the axial stress as a function ofazimuth and radial distance from the center of the borehole. These canbe used in conjunction with known methods to predict the possibility offailure prior to the deformation of the casing approaching the failurepoint.

Another embodiment of the disclosure measures changes in magneticpermeability as a stress indicator. A suitable tool for the purpose isdisclosed in FIGS. 7A, 7B. The tool is provided with an axially orientedexciter coil 740 on a suitable location of the mandrel 34 and the pad(s)are provided with one or more transversely oriented receiver coils 701a, . . . 701 n. With a plurality of such pads, it is possible to get afull 360° azimuthal coverage of the casing in the borehole. The excitercoil 740 is driven at a preselected low frequency and the voltageinduced in the receiver coils 701 a, . . . 701 n is related to themagnetic permeability of the casing near the receiver coil. Receivercoil voltage logs are run to give an indication of the changes in themagnetic permeability of the casing with azimuthal direction. Theprinciple behind this has been discussed in U.S. Pat. No. 4,708,204 toStroud and requires the measurement of the induced current in an axiallyoriented receiver coil in both an unstressed and a stressed condition.

As noted in Stroud magnetic flux induced into the drill pipe by an inputsignal will produce eddy currents which will in turn create anelectromagnetic field. This secondary magnetic field produced by eddycurrent flow in the casing may be detected by a receiving coil. If theinput signal as well as all other variables are held constant then thesignal on the receiving coil will vary in amplitude and phase as afunction of the magnetic permeability of the pipe. As discussed inStroud, stress changes in the casing result in changes in magneticpermeability which, in turn, affects the induced current in an axiallyoriented coil. In the present disclosure, the transversely orientedcoils provide an indication of azimuthal stress changes in magneticpermeability. An azimuthal plot of the magnetic permeability can thuspinpoint directions of maximum and minimum principal stress, and themagnitude of the difference between the maximum and minimum permeabilityis an indication of the difference between the maximum and minimum axialstress. This method does not require measurements made in an unstressedcondition. The method can thus be used to identify imminent casingfailure as discussed with reference to FIGS. 5-6B. It is anticipatedthat the embodiments shown in FIGS. 7A, 7B would have an operatingfrequency in the range of 100-500 Hz, though this is not to be construedas a limitation. Similarly, the embodiment disclosed in FIG. 7C,discussed next, is expected to operate in a frequency range of 500 Hz to5 KHz.

Turning next to FIG. 7C, an arrangement in which both the exciter coiland the receiver coil are transversely oriented is shown. In principle,the received signal with both the exciter and the receiver beingtransversely oriented will have a higher signal level than one in whichonly the receivers are transversely oriented (as in FIG. 7B). Inaddition, the distance between the exciter coils 721 a . . . 721 n andthe receiver coils 731 a . . . 731 n is less than the distance betweenthe exciter coil 740 and the receiver coils 701 a . . . 701 n. This toowill give an improved signal level. A potential drawback is the need tohave the exciter coils 721 a . . . 721 n in FIG. 7C properly calibrated.

Turning now to FIGS. 8A-8B, another embodiment of the disclosure isshown. In this case, the sensor pad 40 is provided with a plurality ofsensors 801 a, . . . 801 n. An axial cross-section of a sensor is shownin FIG. 8B. The sensor comprises a magnet 803, the poles of which are incontact with the formation by ferromagnetic brushes. As the tool isaxially pulled up the borehole, the magnetic flux density in the casingwall immediately below the brush 807 b is less than in the region of thecasing between the brushes 808 a, 807 b. There is a significant changein the magnetic flux density directly below the brush 807 b. As the toolis moved, this results in a changing direction of magnetic field thatproduces a reorientation of magnetic domains in the casing. This resultsin Barkhausen noise. The Barkhausen noise is detected by an inductive orother type of magnetic field sensor 805 positioned in a vicinity of thebrush 807 b in the casing. The Barkhausen noise level is measured as afunction of azimuth, and the azimuthal variation is related to azimuthalchange in the magnetic permeability and azimuthal change in the axialstress in the casing. For the purposes of the present disclosure, themoving permanent magnet arrangement 803, 807 a, 807 b may be regarded asa transmitter that reorients magnetic domains in the casing and themagnetic field sensor 805 is a receiver responsive to changes in themagnetic field.

For each of the embodiments disclosed in FIGS. 6-8, knowing the elasticproperties of the casing and the variation in acoustic or magneticproperties with stress, it is possible to use a table lookup to estimatethe bending stress to which the casing is being subjected, and thus takeappropriate remedial action to avoid casing failure.

The first embodiment of the disclosure has been described above is aspecific example of using EMATS as the acoustic sensors. This is not tobe construed as a limitation on the disclosure. The method of thedisclosure could also be carried out using other side types of sensorssuch as piezoelectric transducers and wedge transducers. Wedgetransducers are discussed, for example, in U.S. Pat. No. 4,593,568 toTelford et al.

The disclosure has been described above with reference to a deviceconveyed on a wireline. However the method of disclosure may also bepractices using the tool conveyed on a tubular such as a drillstring orcoiled tubing, or on a slickline.

Implicit in the processing method of the present disclosure is the useof a computer program implemented on a suitable machine readable mediumthat enables the processor to perform the control and processing. Themachine readable medium may include ROMs, EPROMs, EAROMs, Flash Memoriesand Optical disks. Such a computer program may output the results of theprocessing, such as the stress constraints, to a suitable tangiblemedium. This may include a display device and/or a memory device.

What is claimed is:
 1. An apparatus for determining an azimuthal stressvariation in a tubular within a borehole, the apparatus comprising: alogging tool including at least one transducer configured to generate awave in the tubular; a plurality of acoustic receivers azimuthallydistributed in the logging tool, each of the acoustic receiversconfigured to produce a signal responsive to the generated wave and anazimuthal stress variation in the tubular; and at least one processor;and instructions on a non-transitory medium accessible to the at leastone processor; wherein the at least one processor is configured toexecute the instructions to determine from the signal from each of theplurality of acoustic receivers an indication of the azimuthal stressvariation in the tubular.
 2. The apparatus of claim 1 wherein the atleast one processor is further configured to estimate from theindication of the azimuthal stress variation a value of a shear stressin the tubular.
 3. The apparatus of clam 1 wherein each of the pluralityof acoustic receivers further comprises an electromagnetic receivermounted on a pad extendable from a body of the logging tool and whereinthe signal produced by each of the plurality of electromagneticreceivers is indicative of a magnetic permeability of the tubular. 4.The apparatus of claim 1 wherein the at least one transmitter furthercomprises a plurality of electromagnetic transmitters mounted on a padextendable from a body of the logging tool.
 5. The apparatus of claim 1wherein the at least one transmitter comprises a permanent magnetconfigured to reorient a magnetic domain in the tubular and wherein theat least one receiver comprises a magnetic flux sensor responsive to thereorientation of the magnetic domain.
 6. The apparatus of claim 1wherein the at least one transmitter and the plurality of acousticreceivers are selected from the group consisting of: (i)electro-magnetic acoustic transducers, (ii) piezoelectric transducers,and (iii) wedge transducers.
 7. The apparatus claim 5 wherein theplurality of acoustic receivers are disposed on at least one padextendable from a body of the logging tool.
 8. The apparatus of claim 1wherein the at least processor is further configured to use a tablelookup to estimate the value of the shear stress from the indication ofthe azimuthal stress variation.
 9. The apparatus of claim 1 furthercomprising a conveyance device configured for conveying the logging toolinto the borehole, the conveyance device selected from (i) a wireline,(ii) a drilling tubular, (iii) a slickline, and (iv) coiled tubing. 10.The apparatus of claim 1 wherein the tubular is selected from the groupconsisting of: (i) production tubing, (ii) casing, and (iii) a drillingtubular.
 11. An apparatus for determining an azimuthal stress variationin a tubular within a borehole, the apparatus comprising: a logging toolincluding at least one transducer configured to generate a wave in thetubular; a plurality of magnetic field sensors azimuthally distributedin the logging tool, each of the magnetic field sensors configured toproduce a signal responsive to an azimuthal stress variation in thetubular and at least one of: (i) a Barkhausen noise produced by a magneton the tool and (ii) a current in an axially oriented transmitter coil;and at least one processor; and instructions on a non-transitory mediumaccessible to the at least one processor; wherein the at least oneprocessor is configured to execute the instructions to determine fromthe signal from each of the plurality of magnetic field sensors anindication of the azimuthal stress variation in the tubular.